Vascular diseases are often manifested by reduced blood flow due to atherosclerotic occlusion of vessels. For example, occlusion of the coronary arteries supplying blood to the heart muscle is a major cause of heart disease. Invasive procedures for relieving arterial blockage such as bypass surgery and balloon dilatation with a catheter are currently performed relying on estimates of the occlusion characteristics and the blood flow through the occluded artery. These estimates are based on measurements of occlusion size and/or blood flow or blood pressure before and after the stenosis. Unfortunately, current methods of occlusion size and blood flow measurement have low resolution, are inaccurate, are time consuming, require expertise in the interpretation of the results and are expensive. Thus, decisions on whether or not to use any of the blockage relieving methods and which of the methods should be used are often based on partial information. The evaluation of therapeutic success is also problematic, where both occlusion opening and stent position must be evaluated.
Typically, the physician first selects the appropriate treatment method from among medication therapy, transcatheter cardiovascular therapeutics (TCT), coronary artery bypass grafting (CABG), or non-treatment. Atherosclerotic lesions may have different characteristics. Some lesions exhibit a variable degree of calcification while others have a fatty or thrombotic nature. Lesion characteristics together with vessel condition distal to the lesion and the vascular bed (VB) condition are the major factors for determining the therapeutic procedure needed. Recently, increasing numbers of patients are directed toward TCT. TCT starts with an interventional diagnosis procedure (most commonly used in angiography), followed by the treatment of the patient with medication therapy, CABG or continuation of the TCT procedure with adequate interventional treatment. TCT final stage include diagnosis tools, for the evaluation of treatment success.
Numerous methods are currently available for treating various lesion types. Some of these methods are given herein below, sequenced from "softer" to "heavier", relating to their ability to open calcified lesions; percutaneous transluminal angioplasty (PTCA), "Cutting balloon" angioplasty, directional coronary atherectomy (DCA), rotational coronary atherectomy (RCA), Ultrasonic breaking catheter angioplasty, transluminal extraction catheter (TEC) atherectomy, Rotablator atherectomy, and excimer laser angioplasty (ELCA). Often, stents are placed within the lesion so as to prevent re-closure of the vessel (also known as recoil). If the stent is malpositioned, it disrupts the flow and may initiate restenosis.
Lesion characteristics, together with vessel condition proximal and distal to the lesion and vascular bed condition are used to determine the medically and economically optimal treatment method or combination of methods of choice. The main geometrical parameter of the lesion is stenosis severity As/Ao. Here As is the minimal open cross-sectional area of the stenosis and Ao is the nominal cross-sectional area of the unobstructed vessel. The second parameter is the stenosis length. Another clinically important lesion characteristic is the lesion calcification level. A non-calcified arterial wall or lesion is usually a non-chronic, fat based plaque that may be treated by medication therapy, or by the softer, less expensive, PTCA method. Heavily calcified lesion typically requires harder methods, such as ELCA. The calcification level influences the decision whether to use a dilatation balloon prior to stenting. For example, in cases of very soft lesions, the physician may elect not to use a dilatation balloon prior to stenting. In cases where the degree of calcification dictate the use of such a balloon, the vessel wall calcification level influences the optimal inflation pressure of the dilatation balloon. Chapter 12 entitled "CALCIFIED LESIONS" of the book "The New Manual of Interventional Cardiology" (Eds. Mark Freed, Cindy Grines and Robert D. Safian, Physicians' Press, Birmingham, Mich., 1996, pp. 251-261), discusses various methods for the assessment of the degree of vessel wall calcification and their importance in selecting a treatment method.
Decisions about post dilatation processes such as stent deployment for preventing wall recoil and restenosis, or radiation exposure for preventing restenosis caused by cell proliferation, are also influenced by vessel wall and lesion characteristics. Unfortunately, while lesion geometry is evaluated by angiography, qualitative coronary angiography (QCA), or by intravascular ultrasound (IVUS), accurate information regarding the vessel wall structure and composition and the degree of calcification of the lesion and of the vessel wall sections neighboring the lesion is frequently unavailable due to the expenses involved in obtaining this information. Angiography has been the main diagnostic tool in the cath lab. The physician interprets angiographical images in the following sequence: identification and location of the severe lesions, evaluation of the occlusion level (in diameter percentage of the occluded portion), qualitative estimation of the perfusion according to "thrombolysis in myocardial infarction" (TIMI) grades, determined according to the contrast material appearance. TIMI grades 0, 1, 2, 3 represent no perfusion, minimal perfusion, partial perfusion and complete perfusion, respectively.
Among the more sophisticated diagnostic tools are qualitative coronary angiography (QCA), intravascular ultrasound (IVUS), intravascular Doppler velocity sensor (IDVS) and intravascular pressure sensor (IPS). QCA calculates geometrical properties from angiographic images, in image zones that are chosen by the physician. IVUS provides accurate geometrical data regarding cross section and accurate information regarding the vessel wall structure and composition. Physiological parameters have been introduced in order to help the clinician to elect the appropriate clinical solution. IDVS provides velocity measurements, enabling discriminating various degrees of occlusion according to coronary flow reserve (CFR) criteria. IDVS suffers from inaccuracy problems resulting from positioning error within the vessel.
IPS provides pressure measurements enabling discriminating various degrees of occlusion according to the FFR (fractional flow reserve) criteria and according to the pressure drop across the stenosis. While measuring the pressure based parameters, the transducer should cross the stenosis and measure pressure downstream of the stenosis. The need to cross the stenosis prevents the use of this parameters for purely diagnostic purposes, since stenosis crossing is considered of high risk and therefore, unjustified for diagnostic purposes.
Angiography and the sophisticated techniques discussed above may be employed prior to and after therapeutic procedure (the last for the evaluation of the results and decision about correcting actions). unfortunately, the above discussed sophisticated methods are rarely used due to their high price, operation complexity and the prevailing feeling among physicians that while they provide more accurate information, this information usually does not contribute to clinical decisions.
Pressure, flow and geometry are three variables often measured in the cardiovascular system. Recent progress in invasive probe miniaturization, improvements of the frequency response of probe sensors and computerized processing have opened a whole new range of intravascular pressure and flow measurements and analysis that have been previously impossible to perform. A method for determination of the reflection sites in the arterial system was suggested by Pythoud, F. Stergiopulos, N. Westerhof, N. and Meister, J. J. in "Method for determining distributions of reflection sites in the arterial system" in Am. J. Physiol 271 (1996). They studied reflections of pressure and flow waves generated by the beating heart, in the arterial tree using simultaneous pressure and flow measurements. The low (up to 10 Hz) bandwidth of the pressure and flow signals prevent accurate determination of the distance to reflection site by these authors. Correct determination of reflection source location requires accurate estimation of the pressure wave velocity (PWV) in the vessels under consideration and under the specific pressure signal, in contrast with literature data that is based on healthy arteries under beating heart pulses. All known methods for PWV measurement, used two, three or more simultaneous measurements, which prove impractical considering clinically available tools and methods. Further, various attempts have been done to analyze pressure and flow wave changes caused by occluded sites. Harmonic distortions, changes in pressure wave velocity phase velocity, wave attenuation, and additional reflection sites within the arterial tree prevent successful interpretation and implementation within clinical methods or tools.